bug: S3 multipart upload doesn't respect content encoding

[dentiny]

Describe the bug

According to the doc, Content-Encoding is part of the request header.

It seems to be set for plain PUT: here and here

But I don't see the field set for multipart upload initialization https://github.com/apache/opendal/blob/main/core/services/s3/src/core.rs#L785

Steps to Reproduce

N/A

Expected Behavior

Both plain put and multipart upload should have content encoding properly set.

Additional Context

No response

Are you willing to submit a PR to fix this bug?

• Yes, I would like to submit a PR.

[dosubot]

Thanks for the detailed bug report — your analysis is correct! The Content-Encoding header is indeed missing from the s3_initiate_multipart_upload function (lines 810-862), while plain PUT requests properly set it via insert_metadata_headers.

This is actually part of a recurring pattern where headers work for single-part uploads but are missing from multipart uploads:

• Issue #7254: x-amz-acl header was missing from multipart upload
• Issue #5429: User metadata not set for multipart uploads

For your PR, you'll want to add the Content-Encoding header in s3_initiate_multipart_upload, following the same pattern used for Content-Type, Content-Disposition, and Cache-Control in that function:

if let Some(encoding) = args.content_encoding() {
    req = req.header(CONTENT_ENCODING, encoding);
}

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